FR3055328A1 - PROCESS FOR THE PRODUCTION OF SYNTHESIS GAS USING A STEAM METHANE REFORMING UNIT - Google Patents

Abstract

A synthesis gas production process employing a steam methane reforming unit (SMR) comprising: - the addition of oxygen to the combustion air stream feeding the burners of the SMR unit and - the implementation of a reforming furnace at least a portion of the walls is covered by a refractory coating to improve the overall emissivity coefficient of the walls to a value greater than 0.4.

Description

Holder (s): AIR LIQUIDE, ANONYMOUS COMPANY FOR THE STUDY AND EXPLOITATION OF GEORGES CLAUDE PROCESSES Société anonyme.

Extension request (s)

Agent (s): AIR LIQUIDE.

FR 3 055 328 - A1 (64) PROCESS FOR THE PRODUCTION OF SYNTHETIC GAS USING A STEAM REFORMING UNIT.

©) Process for the production of synthesis gas using a steam methane reforming unit (SMR) comprising:

- the addition of oxygen to the combustion air flow supplying the burners of the SMR unit and

- The use of a reforming furnace, at least part of the walls of which is covered by a refractory coating making it possible to improve the overall emissivity coefficient of the walls to a value greater than 0.4.

3O5532S

The present invention relates to a process for the production of synthesis gas using a steam methane reforming unit.

Steam reforming (in English steam methane reforming or SMR) makes it possible to produce synthesis gas, a mixture composed mainly of hydrogen and carbon monoxide from a gaseous charge of reactants consisting essentially of hydrocarbons and water vapor which react together in a catalytic tubular reactor. This technology, one of the most used for the production of hydrogen in particular, is based on catalytic reactions at high temperature (800-950 ° C) of light hydrocarbons with water vapor. Strongly endothermic, these reactions require heat.

This heat is usually provided by the combustion of a fuel with air using burners located in a radiant oven in which the reforming tubes are placed. The fumes from combustion circulate outside the tubes in the oven and provide the reactants, by radiation and convection, with the heat necessary for reforming.

The reformers we are considering here are steam reformers of usual geometry. The ovens have a certain number of burners arranged in rows on side walls in the case of SMRs called “side-fired” and SMRs called “terrace wall”, or at the level of the roof of the oven in the case of SMRs called “ top-fired ”; more rarely, the burners are placed in the oven floor in the case of "bottom-fired". These different types of burners are shown in Figure 1.

In order to improve the process and in particular to improve the generation of heat, a solution is to add oxygen to the flow of combustion air.

However, this improvement is limited by the increase in the temperature of the combustion gases which is associated with an increase in the production of NOx.

However, the emission of NOx is currently regulated and the devices for reducing

NOx are expensive.

On the basis of this, a problem which arises is to provide an improved SMR process, that is to say one which does not have the drawbacks linked to the increase in the production of NOx.

A solution of the present invention is a process for the production of synthesis gas using a steam methane reforming unit (SMR) comprising:

- the addition of oxygen to the combustion air flow supplying the burners of the SMR unit and

- The implementation of a reforming furnace of which at least part of the walls is covered by a refractory coating making it possible to improve the overall emissivity coefficient of the walls to a value greater than 0.4.

Note that the walls of the furnace can be made of fibers and or refractory bricks and have a basic emissivity coefficient of the order of 0.25.

The emissivity coefficient corresponds to the radiative flux emitted by a surface element (in the context of the invention with regard to the walls of the radiant oven) at a given temperature, compared to the reference value that is the radiative flux emitted by a black body at the same temperature. This last value being the maximum possible value, the emissivity coefficient of a wall will always be a surface property between 0 and 1.

The coating is generally a refractory paint (mixture for example of metal oxides with an alumino-silicate or a silica rich in iron) applied by spraying via for example the use of an industrial gun. The thickness of the refractory lining will be less than a millimeter.

Such a coating improves the radiation of the walls favoring the transfer of heat between the walls of the oven.

Depending on the case, the method according to the invention may have one or more of the following characteristics:

- the refractory coating makes it possible to improve the overall emissivity coefficient of the walls to a value greater than 0.65.

the addition of oxygen is such that the oxygen content in the air flow is between 20.5% and 23.5%,

- at least one third of the walls of the furnace is covered by the refractory lining,

- the third of the walls of the furnace having the refractory lining corresponds to the third of the walls closest to the burners,

- the heat released by the burners is between 1.0 MW and 3.0 MW for an oven with burners arranged at the top of the oven and between 0.25 MW and 0.75 MW for an oven with burners arranged in rows on the side walls,

- the refractory lining has a thickness of less than a millimeter,

the refractory coating is chosen from among aluminosilicates, silicas rich in iron oxide, preferably doped with metal oxides (Ti, Ni, Cr, Co or other transition metals),

the reforming furnace comprises reforming tubes,

the reforming tubes are made of a material of the HP Alloy type rich in Cr and Ni (for example Manaurite sold by Manoir Industries) and preferably manufactured traditionally by centrifugation,

the addition of oxygen is carried out by adding a flow comprising between 90% and 100% of oxygen.

The method according to the invention associates the oxy-combustion, that is to say the addition of oxygen in the flow of combustion air, and the refractory lining on at least part of the walls of the furnace SMR.

This association can be applied for ovens comprising a certain number of 20 burners arranged in rows on side walls in the case of SMR called "side-fired" and SMR called "terrace wall", or at the level of the roof of the oven in the case of “topfired” SMRs; it can also be applied for ovens with burners placed in the oven floor in the case of "bottom-fired".

Subsequently for the sake of simplicity, unless otherwise indicated, all the quantifications will be given for ovens comprising burners arranged in rows on side walls, that is to say for so-called "side-fired" ovens.

FIG. 2 illustrates a method according to the invention for a side-fired type oven. The feed gas containing mainly methane and water vapor is sent to catalytic tubular reactors. In order for the mixture to react and produce synthesis gas, containing mainly hydrogen and carbon monoxide, heat is provided by the combustion of at least one fuel, the most common of natural gas, associated in the majority of units use recycled gas from synthesis gas purification systems. Air is drawn from the atmosphere and thanks to its oxygen content, below the

21%, maintains combustion. The air can be preheated before being sent to the burners, either in the heat recovery system from the flue gases, as illustrated in the figure, or the synthesis gas.

According to the invention, the reforming unit will have at least a portion of the walls of the oven coated with a high-emissivity paint and the combustion air will be enriched with oxygen which can wait for a content of 23.5%.

Note that the method according to the invention can be implemented in new installations but also in old installations during a maintenance period. The addition of oxygen is limited to 23.5% in order to use the current technology of the air blower and to avoid the additional safety constraints linked to the basic equipment of the oxygen flow and to the oxygen handling.

Thanks to the process according to the invention, by keeping the flow rate of synthesis gas constant, the consumption of synthesis gas will decrease. Indeed the addition of oxygen makes it possible to reduce the consumption of natural gas.

Another possibility is offered by the invention: keep the same consumption of natural gas and obtain a higher flow rate of synthesis gas.

Also thanks to the invention, the operator will have the choice between obtaining a higher flow rate of synthesis gas or reducing his consumption of natural gas.

For an addition of oxygen such that the oxygen content in the air flow increases from 20.5% to 23.0%, one obtains either a natural gas consumption reduced by 7% or a production of synthesis gas increased by 6 %.

More specifically, the advantages of adding oxygen to the air flow are:

- the reduction in the quantity of nitrogen in the combustion gases, hence the reduction in the quantity of heat losses by the combustion gases and therefore the increase in the heat available for the process,

- an increase in the adiabatic flame temperature of the order of + 100 ° C and an increase in the emissivity of the combustion gases due to the higher concentrations of CO ₂ and H ₂ O, and consequently an improvement in the transfer heat to the reforming tubes.

Table 1 shows that the addition of oxygen to the flow of combustion air leads to an increase in the adiabatic temperature of the flame of a burner supplied with natural gas.

Process without addition oxygen Process with addition oxygen % mol O ₂ in air 20.6% 23.5% Adiabatic flame temperature 2407 ° C 2526 ° C

Table 1

Note that in the process according to the invention less CO ₂ is released into the atmosphere 10 due to less consumption of natural gas.

From an economic point of view even if the addition of oxygen to the combustion air stream has a cost, the excess heat being available for the process means that the overall operating cost is reduced and savings can be obtained.

Regarding the impact of the refractory lining, remember that in an SMR oven, the convective heat transfer is negligible compared to the radiation heat transfer (approximately 95% of the heat transferred during an SMR process is due to a phenomenon of radiation).

The heat transfer by radiation from the wall of the furnace in the tubes is due to the refractory temperature and to the properties of radiation such as the emissivity which can be improved by the application of a paint on the walls surrounding the furnace.

Thanks to the refractory lining, the heat flow transferred to the reforming tubes is improved, which consequently increases the yield of the reforming reaction and therefore the rate of production of synthesis gas.

Concerning the impact of the combination of oxygen addition in the air flow 25 refractory lining is quantified thanks to a specific one-dimensional software composed of two coupled parts: a combustion chamber model on one side and tubes of reforming on the other side.

The comparisons between the process according to the invention and the processes with only addition of oxygen or only the implementation of the refractory lining or the basic processes without addition of oxygen or the implementation of a refractory lining are carried out for a "side-fired" furnace with the assumption of a constant supply, a constant temperature of the combustion air, a constant excess of oxygen in the combustion gases and a constant temperature of the synthesis gas .

Figure 3 shows that the combination of adding oxygen to the refractory lining air stream 10 improves heat transfer to the reforming tubes. This improvement is shown regardless of the operating mode chosen: mode with reduction in the consumption of natural gas due to the addition of oxygen (Figure 3a) or mode with increase in the production of synthesis gas (Figure 3b).

Better heat transfer to the reforming tubes leads to a gain in efficiency of the furnace which can be upgraded according to the operator's desire: to produce more synthesis gas or reduce the consumption of natural gas. The efficiency gains for the two operating modes are summarized in Table 2. In the example chosen to illustrate the invention, the addition of oxygen has a greater impact when the oxygen content is increased to 23%. in comparison with the addition of a refractory coating applied to the entire walls of the furnace when the emissivity coefficient initially at 0.24 drops to 0.61. It is observed in the last line of Table 2 that the gains brought by the addition of oxygen and the refractory lining accumulate.

Goals Increase in synthesis gas production Reduction of consumption of natural gas Hypotheses A = Refractory coating applied to all the walls of the oven (emissivity coefficient increased from 0.24 to 0.61) 3.02% 2.79% B = Addition of oxygen to the air flow (passage of the oxygen content from 20.5% to 23%) 5.97% 6.84% Combination of A + B 9.19% 10.01%

Table 2

The benefit of adding oxygen can be limited by the increase in the temperature of the flue gases (Figure 4) which is associated with an increase in NOx production inside the furnace, especially when a an increase in the production of synthesis gas is requested (Figure 4a). The increase in the temperature of the combustion gases for a process in “reduction of natural gas consumption” mode is shown in FIG. 4b.

By combining the addition of oxygen into the air flow and the refractory lining of at least part of the walls of the oven as mentioned in the process according to the invention, the temperature of the combustion gases in the oven is controlled and always remains below the reference value. By “reference value” is meant a basic process without the addition of oxygen or the use of a refractory lining.

Consequently, the solution according to the invention overcomes the need for an expensive NOx reduction device.

Furthermore, the addition of oxygen in the air flow not combined with the refractory lining causes an increase in the temperature of the walls of the furnace, in particular in the regions where the heat of the burner is released (FIG. 5); these regions will then be weakened in the long term. Note that this drawback can be overcome by combining the addition of oxygen into the air flow and the refractory lining of at least part of the walls of the furnace as mentioned in the process according to the invention. This is due to the fact that the refractory lining considerably reduces the temperature of the walls of the furnace, and consequently the heat losses through the walls (cf. FIG. 6).

Overall, by combining the addition of oxygen in the air flow and the refractory lining of at least part of the walls of the furnace as mentioned in the process according to the invention, less heat loss is observed through the walls. of the furnace only in a process using only the addition of oxygen to the air flow.

The maximum operating temperature of the reforming tubes included in the reforming furnace depends on several factors, in particular the mechanical load of the tubes, the mechanical properties of the alloys used for the tubes and the desired lifetime of the tubes exposed to creep and thermal aging.

Any intensification of the heat transferred to the tubes has a direct positive impact, either by increasing the productivity or by improving the compactness of the combustion chamber which is precious in terms of expenditure. However, the intensification of the transferred heat must not lead to excessive skin temperatures for the reforming tubes (for example temperatures above 1000 ° C.). Indeed, such excessive skin temperatures lead to a reduction in the life of the reforming tubes or require the use of more resistant alloys which would be much more expensive.

By combining the addition of oxygen in the air flow and the refractory lining of at least part of the walls of the furnace as mentioned in the process according to the invention, an increase of less than 10 ° C. is observed. temperature of the tubes, regardless of the operating mode chosen: reduction in the consumption of natural gas or increase in the production of synthesis gas.

Table 3 summarizes the qualitative advantages of the combination of the addition of oxygen in the air flow and the refractory lining of at least part of the walls of the furnace as mentioned in the process according to the invention.

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A = Refractory lining on at least part of the oven walls B = Addition of oxygen to the air flow Combination from A + B Benefits of combination Efficiency gain Improved Improved Improved Accumulation of improvement Heat transfer to tubes Increases Increases Increases Accumulated increase not Flue gas temperature Scaled down Increase in burner area scaled down No more NOx regulation problem Oven wall temperature Increased Scaled down Scaled down Oven wall life Heat loss through the oven walls Reduced Increased Reduced Gain efficiency Maximum tube temperature Oven with burners on the top of the oven Scaled down Increased Scaled down Lifetime reforming tubes is checked Oven with burners on the oven walls Increased Increased Increased Increased

Table 3 ίο

Claims (7)

Claims 1. Process for the production of synthesis gas using a steam methane reforming unit (SMR) comprising: - the addition of oxygen to the combustion air flow supplying the burners of the SMR unit and - The implementation of a reforming furnace of which at least part of the walls is covered by a refractory coating making it possible to improve the overall emissivity coefficient of the walls to a value greater than 0.4. 2. Method according to claim 1, characterized in that the refractory coating makes it possible to improve the overall emissivity coefficient of the walls to a value greater than 0.65. 3. Method according to one of claims 1 or 2, characterized in that the addition of oxygen is such that the oxygen content in the air flow is between 20.5% and 23.5%. 4. Method according to one of claims 1 to 3, characterized in that at least one third of the walls of the furnace is covered by the refractory lining. 5. Method according to claim 4, characterized in that the third of the walls of the furnace having the refractory lining corresponds to the third of the walls closest to the burners. 6. Method according to one of claims 1 to 5, characterized in that the heat released by the burners is between 1.0 MW and 3.0 MW for an oven with burners arranged at the level 25 from the roof of the oven and between 0.25 MW and 0.75 MW for an oven with burners arranged in rows on the side walls. 7. Method according to one of claims 1 to 6, characterized in that the refractory lining has a thickness less than a millimeter. 8. Method according to one of claims 1 to 7, characterized in that the refractory coating is chosen from aluminosilicates, silicas rich in iron oxide, preferably doped with metal oxides. 5 9. Method according to one of claims 1 to 8, characterized in that the reforming furnace comprises reforming tubes. 10. The method of claim 9, characterized in that the reforming tubes are made of a material of the HP Alloy type rich in Cr and Ni and preferably manufactured 10 traditionally by centrifugation. 11. Method according to one of claims 1 to 10, characterized in that the addition of oxygen is carried out by addition of a flow comprising between 90% and 100% of oxygen. 1/5 Topfired Bottom fired Sidefired Terrace wall 1 1 1 h P ' —— ' O ? 1 C J Lr y L 1 ] 1 - - ÿ "